We have developed an in vitro system that synthesizes bacteriophage phi X-174 single-stranded DNA and the synthesized DNA is encapsidated into a prohead to form infectious phage particles. We have purified phage-coded proteins necessary for ssDNA synthesis. These are prohead structure-containing genes B, D, F, G and H proteins, gene J protein, which is a phage structural protein not found in the prohead, gene A protein, a nicking and closing protein of ssDNA, and gene C protein which is needed for "activation" of replicative form DNA to serve as the template of the system. These purified proteins with protein fractions from uninfected cell extract are capable of synthesizing ssDNA and assembling infectious phage. Using the system, we are studying biochemistry of phage-coded and host-originated proteins participating during the morphogenetic process.